The present invention relates to an apparatus for molding a honeycomb structure and a method for molding a honeycomb structure using the molding apparatus. More particularly, it relates to a molding apparatus for making a honeycomb structure useful, for example, as a catalyst carrier for controlling exhaust gases of automobiles, and a method for molding the honeycomb structure which makes it possible to reduce failure in formation of outer walls of the honeycomb structure by adding various functions to a press plate which fixes a spinneret.
Problems which become afresh are influences on earth environments and ecosystems of nitrogen oxides, sulfur oxides, hydrogen chloride, etc. contained in exhaust gases of automobiles in addition to carbon dioxide which warms the earth. It is said that, in the future, automobiles which utilize as energy source, for example, electricity, natural gas or methanol which have less influences on the ecosystem will be substituted for automobiles which utilize gasoline and gas oil as energy source. However, there are difficulties to be overcome for practical use of these technologies, and tentative measures are reduction of cost of fuels and purification of exhaust gases. The honeycomb structures with which the present invention concerns are used for exhaust gas purification apparatuses of automobiles as a technology for the solution of environmental problems.
For example, honeycomb structures made of ceramics materials or metallic materials as shown in FIGS. 6-8 are used as catalyst carriers for exhaust gas purification apparatuses of automobiles. FIG. 7 is an oblique view of a honeycomb structure 61, FIG. 8 is a front view of the honeycomb structure 61, and FIG. 6 is a partially enlarged view of the honeycomb structure 61. This honeycomb structure is nearly columnar as shown in FIG. 7, and comprises partition walls 64 having a honeycomb structure which form many cells 63 and an outer wall 62 covering the outer periphery.
For making such honeycomb structure 61, an apparatus for molding honeycomb structures, for example, as shown in FIG. 5 is used. FIG. 5 is a vertical sectional view of an apparatus 50 for molding honeycomb structure which is provided with back pore part 53 from which a molding material is introduced, a spinneret 54 having slit a part 52 which ejects the molding material, and a press plate 55 provided downstream the spinneret 54. Extrusion molding is carried out using this apparatus to make the honeycomb structure 61.
In the honeycomb structure molding apparatus 50, the spinneret 54 comprises inner side part 71 and outer periphery part 72, and the inner side part 71 protrudes to downstream side to form a level difference part 75 between the inner side part 71 and the outer periphery part 72. At the press plate, there is provided a gap part 57 which molds the outer wall of the honeycomb structure. Press jig 58 and back press plate 59 are holders for setting the spinneret 54 and the press plate 55.
In carrying out extrusion molding by the honeycomb structure molding apparatus, the molding material is extruded from the upstream side of spinneret 54 to the downstream side through the spinneret 54 by an extruder (not shown) as shown in FIG. 5. The molding material extruded from slit 73 provided in the inner side part 71 of the spinneret 54 opened on the downstream side forms a honeycomb structure comprising many cells 63.
On the other hand, the molding material extruded from slit 74 provided in the outer peripheral part 72 of the spinneret 54 changes in its running direction to the level difference part 75 from the extrusion direction with rupturing of the honeycomb shape being caused by the action of the slit part 57, and again changes in its running direction to the extrusion direction at the position where the press plate 55 opens, thereby forming an outer wall 62 surrounding the cells 63.
However, in such conventional honeycomb structure molding apparatus and molding method, there is the problem of failure in formation of the outer wall.
As an example of failure in formation of outer wall, mention may be made of formation of waviness as shown in FIG. 9 and FIG. 10. FIG. 9 is a front view of a honeycomb structure having wavy wall, and FIG. 10 is a side view of a honeycomb structure having wavy wall. It is considered that wavy wall portion 92 of the honeycomb structure 91 is formed because the amount of the molding material becomes larger than in other portions and cells are ruptured to form waviness, and it is assumed that this is caused since molding speed of the outer wall part is higher than that of the part of the cells 63.
If this wavy wall is formed at various portions of the outer wall, failure in bending is caused. The honeycomb structure which is to be columnar is formed with undulation, resulting in increase of gas permeability resistance in addition to inferior appearance and thus the honeycomb structure is not suitable as product.
As another example of failure in formation of the outer wall, there is the formation of burrs as shown in FIG. 11 and FIG. 12. FIG. 11 is a front view of a honeycomb structure having burrs, and FIG. 12 is a side view of a honeycomb structure having burrs. It is considered that the burr portions 96 of the honeycomb structure 95 are formed because the amount of the molding material becomes smaller than in other portions and the outer wall is not formed, and it is assumed that this is caused since molding speed of the outer wall portion 62 is lower than that of the portion of the cells 63.
It is considered to be inevitable that the molding speed of the portion of the outer wall 62 becomes higher or lower than that of the portion of the cells 63, since fluidity of the molding material cannot necessarily be maintained constant and there is variation in working precision of the spinneret or the press plate. The fluidity of the molding material is affected by various factors. For example, in the case of ceramics, the fluidity is affected by delicate variations of particle diameter or shape of the molding material, water content, kneading degree, atmospheric temperature and humidity, etc. Therefore, the fluidity of the molding material changes momentarily even in a day, and can hardly be maintained constant.
Thus, change of fluidity of the molding material greatly affects the extrusion moldability. Therefore, it is necessary to select optimum shape or size of the press plate 55 from a plurality of press plates before starting of the extrusion molding or to change thickness of the slit part 57. It is also necessary to select the back press plate 59 of optimum inner diameter upon occasion.
However, even if the press plate 55 and others are set in the optimum state, the fluidity of the molding material is not necessarily stable owing to environmental changes at the use and variation of the molding material, and, as a result, satisfactory outer wall 62 cannot necessarily be formed. Therefore, it is necessary to stop the honeycomb structure molding apparatus 50 each time and exchange the press plate 55 or adjust the back press plate 59. However, these measures are temporary ones to deal with the conditions such as environment under which failure of formation of outer wall occurs, but cannot be employed when the environment changes. Furthermore, these measures require a relatively long operation time, and hence there is the problem of sharp reduction of productivity.
In order to solve the above problems, various means have been proposed.
For example, JP-A-8-239701 discloses a metallic honeycomb body and a method for making the same. FIG. 2 is an elevation sectional view of a die assembly of the disclosed apparatus for making metallic honeycomb bodies. Using the apparatus 20 for making metallic honeycomb materials comprising combination of a mask 22 having a ring reservoir and a die 24 having a skin forming slit 28 of a suitable size, a plasticized batch material discharged outside from slots 23 near the peripheral part of the die 24 is discharged into adjacent but different two areas of the die 24. That is, by discharging the material into the ring reservoir 21 in the closed surface of mask 22 and into the skin forming slit 28 formed between outlet surface and edge of mask opening part, honeycomb (cell portion) and outer skin (outer wall portion) are sufficiently bonded without causing separation, and thus a firmly integrated metallic honeycomb body having a thick outer skin can be obtained.
However, in the case of, for example, a ceramics molding material, it is known that being different from metal particles, the fluidity is greatly different depending on the flow direction of the molding material particles, and failure in formation of outer wall often occurs.
JP-A-9-277234 discloses an apparatus for molding honeycomb structures and a molding method. FIG. 3 is a sectional view of the disclosed apparatus for molding honeycomb structures. The molding apparatus 30 for honeycomb structures comprises a metal mold 33 having a slit 31 for molding the honeycomb structure 32 and a guide ring 34, and the guide ring 34 has an opening edge part 35 and a pool part 36, and, besides, a temperature adjusting device 37 for adjusting the temperature of the guide ring 34. By this temperature adjusting device 37, temperature of ceramic material for forming outer skin (outer wall) which is retained in the pool part 36 of the guide ring 34, and, as a result, fluidity of the ceramic material can be changed and, hence, the molding rate of the outer skin (outer wall) and that of the body portion (cell portion) can be coincided. Thus, formation of wrinkles, burrs, etc. can be inhibited.
However, in the case of using a molding material inferior in heat conduction, sufficient heat cannot be given to the molding material only by the adjustment of temperature and sufficient effect cannot be obtained.